In weighing units (scales) and force sensor assemblies, electromechanical force transducers, also known as load cells, are usually used today for the conversion of the weight force to be determined into an electrical value. This electrical value is then further processed in a suitable form, in order to present the measured value in units of weight in a form which the user can read on a display of the weighing unit or transmitted to monitoring devices. The load cells that are most frequently used are based on the strain gage.
During the operation of a scale or a force measurement system, the load cells are not only loaded with the weight of the item being weighed but also with a constant initial load, which is made up of the weight of the so-called platter of the scale or the force transmitter of the force measurement system, on which the item to be weighed can be placed or the force transmitted through.
One area where load cells and associated electronic force sensors or transducers find applicability is in vehicle automotive and aerospace applications. Generally, it is often desirable to determine the weight and position of an occupant of a seat in a vehicle passenger compartment. For example, based upon the weight of the occupant and the position of the occupant on the seat, an active safety restraint system may determine whether or not to deploy or may determine the amount of force with which to deploy.
Since the development of the air bag and its inclusion in automobiles a problem has existed with the relative deployment force used for various individuals. Air bags have been a requirement on new vehicles since 1992. Air bags are made to arrest the forward momentum of the driver or passenger in the event of a collision. If one designs a universal air bag for all passengers, then it must have sufficient force to stop the largest of the expected passengers. Smaller passengers have less momentum, and so do not require the same momentum change as the larger passenger. In addition, smaller passengers are shorter, and sit closer to the dashboard, and therefore experience more of the bag's explosive force than an average adult male, sitting further back. As a result of the current air bag deployment force, there have been a number of injuries and fatalities associated with air bag deployment. More than a hundred deaths have been attributed to the deployment of air bags with a small adult or a young child, when no air bag deployment would not have resulted in any injury to the occupant.
This situation has caused NHTSA, the National Highway Traffic Safety Administration, a branch of the U.S. Department of Transportation, to propose rules which will change the criteria for air bag activation, as well as the deployment force, in order to protect such small occupants. In addition to these requirements, the NHTSA has also identified “out of position” occupants as a source of concern. Thus a system must be able to modulate or reduce the air bag deployment force if the occupant is in a position so as to be injured by the air bag, even if that occupant is a full size adult.
The automotive industry represents only one area where strain gage force transducers find usefulness. Any application where it is necessary to measure and/or detect force or strain will find use for a strain gage force transducer or strain gage device. Other examples include the aerospace industry and applications such as detecting force on an aircraft body for fatigue cycle studies.
Current load cell and associated electronic force sensor technologies incorporate expensive to harness sensing means such as foil gages, capacitive modules or crystal oscillators. It is difficult to automate the manufacture of such devices. Additionally, many of these devices are not modular in nature. There does not currently exist high output, thick film technology for load sensors that are easy to automate. It is believed that the applications described herein address this important need by providing for a true modular load cell apparatus for use in a variety of electronic force sensor applications.